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setup for SNPL?

 
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brucehughw



Joined: 28 Apr 2013
Posts: 69
Location: Boston, MA, USA

PostPosted: Wed Mar 02, 2016 14:53 pm    Post subject: setup for SNPL? Reply with quote

Hi,

Does anyone know the conditions mostly likely to produce snow and ice pellets?

Thanks, Bruce

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Bruce Wilson
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philnyc



Joined: 23 Sep 2007
Posts: 2819
Location: Brooklyn, New York

PostPosted: Fri Mar 04, 2016 20:15 pm    Post subject: Reply with quote

Do you mean snow mixed with ice pellets?
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Phil Lutzak
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brucehughw



Joined: 28 Apr 2013
Posts: 69
Location: Boston, MA, USA

PostPosted: Fri Mar 04, 2016 22:35 pm    Post subject: Reply with quote

Hi, Phil.

Yes, simultaneous snow and ice pellets, appearing as SNPL in a metar.

thanks, Bruce

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Bruce Wilson
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Frank.D



Joined: 09 Oct 2007
Posts: 291

PostPosted: Thu Mar 17, 2016 15:58 pm    Post subject: Reply with quote

Itís an interesting situation since the ice pellets require a layer of warm air aloft, warm enough (above the melting point of zero C) to melt the falling snow to liquid water, and then fall through a sufficiently deep layer of sub-freezing air to freeze the liquid drops and fall onto the ground as ice pellets. The snow indicates un-melted snowflakes, and so it seems reasonable to guess that the presence of both these precip forms indicates that the location is at the edge of the layer of warm air aloft, where the warm air aloft is marginally warm enough and deep enough, and where mixing and circulation results in local patches of melting conditions mixed with patches of sub-freezing air.
A good example of conditions at the edge of a warm layer!

Cheers
Frank

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brucehughw



Joined: 28 Apr 2013
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Location: Boston, MA, USA

PostPosted: Thu Mar 17, 2016 16:44 pm    Post subject: Reply with quote

Hi, Frank.

Great explanation. And no wonder SNPL is fairly rare.

Thanks, Bruce

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Bruce Wilson
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Frank.D



Joined: 09 Oct 2007
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PostPosted: Fri Mar 18, 2016 18:43 pm    Post subject: Reply with quote

Hi Bruce,
This weekend seems to provide an ideal opportunity to watch this happening in the northeastern snowstorm, where conditions may be appriopriate. A forecast sounding (below) for Sunday afternoon somewhere near eastern New Jersey shows the warm layer aloft just possibly warm enough to melt the snow, while a deep cold layer below could re-freeze precip that melts.
So, Sunday may be an interesting experiment to watch for the precip types reported near the Atlantic coast.

Frank



 Description:
Forecast sounding with warm layer aloft

12_NAM_057_39.94,-73.94_severe_ml.png



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Frank
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brucehughw



Joined: 28 Apr 2013
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Location: Boston, MA, USA

PostPosted: Fri Mar 18, 2016 20:24 pm    Post subject: Reply with quote

Frank,

thanks again. I will watch for that on Sunday (live outside Boston). Curiously enough, I'm working at home this afternoon, and at about 20Z had a great 5-10 minutes of PL and then another bit at about 2015Z. Do you know where I can find a sounding for that time (i.e., in the past)? KBED and KBOS are the closest airports.

Bruce

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Frank.D



Joined: 09 Oct 2007
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PostPosted: Mon Mar 21, 2016 14:18 pm    Post subject: Reply with quote

Bruce,
The main source that I know of for past soundings based on forecast models is the NOAA ARL READY website at ready.arl.noaa.gov where you follow the selections for location, model and time until you can plot a sounding for a particular forecast hour. Attached below is a NAM model sounding for KBOS that I made Monday morning for Friday at 21 UTC.
A source that I find reliable for archive soundings is the U of Wyoming website at http://weather.uwyo.edu/upperair/sounding.html and I tried to plot the archive sounding observation for KOKX for March 19, UTC 00 (but it seems to be unavailable).

Frank



 Description:
NAM sounding March 18 UTC 2100

29183917_profile.gif



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brucehughw



Joined: 28 Apr 2013
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Location: Boston, MA, USA

PostPosted: Mon Mar 21, 2016 15:50 pm    Post subject: Reply with quote

Thanks, Frank.

Nice SN today in the Boston area -- fitting for first day of spring. Very Happy

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Bruce Wilson
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Frank.D



Joined: 09 Oct 2007
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PostPosted: Tue Mar 22, 2016 18:16 pm    Post subject: Reply with quote

The theme of the setup for SNPL continues to be relevant because that's exactly what I am expecting for Thursday morning here in Toronto, westward across the central Great Lakes, and eastward across some northeastern states. The forecast sounding for Toronto from this morning's NAM model is posted below and shows a distinct warm layer above about the 900 mb level (deep enough for snow to melt to liquid) and a distinct sub-freezing layer below that (down to the surface), deep enough for the rain to freeze to IP. Also the sounding has a deep moist layer above the 600 mb level where temperature is colder than -10 deg C and ideal for precipitation to form. Thursday will be an interesting spring day!

Cheers
Frank



 Description:
NAM forecast sounding, Thursday morning, Toronto

NAM_218_2016032212_F48_43.5000N_79.5000W.png



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Frank
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brucehughw



Joined: 28 Apr 2013
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Location: Boston, MA, USA

PostPosted: Tue Mar 22, 2016 18:46 pm    Post subject: Reply with quote

Frank,

Interesting to see if this plays out as you expect. I'll keep on eye out -- probably via Toronto airport (CYYZ) metars. Happy forecasting

Bruce

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Bruce Wilson
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brucehughw



Joined: 28 Apr 2013
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Location: Boston, MA, USA

PostPosted: Wed Mar 23, 2016 21:24 pm    Post subject: Reply with quote

Frank,

could you please tell me where you get those sounding forecasts from. I'd like to be able to enter lat/long and time and get a sounding. I tried COD, but it seems their soundings aren't working - or I'm just not doing it correctly.

thanks, Bruce

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Bruce Wilson
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brucehughw



Joined: 28 Apr 2013
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Location: Boston, MA, USA

PostPosted: Thu Mar 24, 2016 02:08 am    Post subject: Reply with quote

Frank,

I went through some of the 410 material. I'm all set with the forecast soundings.

Thanks, Bruce

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Bruce Wilson
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Frank.D



Joined: 09 Oct 2007
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PostPosted: Thu Mar 24, 2016 14:52 pm    Post subject: Reply with quote

Bruce,
It seems that you got things straightened out with the weather.cod forecast soundings and if so, then I won't elaborate on it except to add a little info for the benefit of Milli Bar Forum readers who may not be aware of these resources, or who have other resources/discussion to share here on the forum.
The twisterdata.com website is also quite handy because you simply click on a spot on the forecast map and then the forecast sounding page includes a basemap with a drag-and-drop pointer to refine the location where you want the sounding.
A good default resource for plotting forecast soundings is the ARL READY website mentioned in an earlier post, ready.arl.noaa.gov and it presents a good sounding image with the option to just plot the layers below 400 mb.
Note that I should have mentioned in an earlier post that the forecast sounding posted for Toronto is a good situation for IP and not for SN/IP since the warm layer might be deep enough to completely melt the snow falling through it. The condition for SN and IP together occurs where the warm layer aloft (above zero deg C) just starts to move over a location, and then snow may fall through the layer as snow while some of the snow may melt (and then re-freeze in the layer above the surface) and then plunk to the ground as IP mixed with snow. This transition occurred at my home (about 50 km east of CYYZ) this morning (and the IP make quite a noise banging against the windows).

Cheers,
Frank

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brucehughw



Joined: 28 Apr 2013
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Location: Boston, MA, USA

PostPosted: Fri Mar 25, 2016 01:19 am    Post subject: Reply with quote

Hi, Frank.

Glad your PL forecast worked out this morning!

Bruce

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